extension.partialclone: introduce partial clone extension

Introduce new repository extension option:
    `extensions.partialclone`

See the update to Documentation/technical/repository-version.txt
in this patch for more information.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/setup.c b/setup.c
index 03f51e0..58536bd 100644
--- a/setup.c
+++ b/setup.c
@@ -420,7 +420,11 @@ static int check_repo_format(const char *var, const char *value, void *vdata)
 			;
 		else if (!strcmp(ext, "preciousobjects"))
 			data->precious_objects = git_config_bool(var, value);
-		else
+		else if (!strcmp(ext, "partialclone")) {
+			if (!value)
+				return config_error_nonbool(var);
+			data->partial_clone = xstrdup(value);
+		} else
 			string_list_append(&data->unknown_extensions, ext);
 	} else if (strcmp(var, "core.bare") == 0) {
 		data->is_bare = git_config_bool(var, value);
@@ -463,6 +467,7 @@ static int check_repository_format_gently(const char *gitdir, int *nongit_ok)
 	}
 
 	repository_format_precious_objects = candidate.precious_objects;
+	repository_format_partial_clone = candidate.partial_clone;
 	string_list_clear(&candidate.unknown_extensions, 0);
 	if (!has_common) {
 		if (candidate.is_bare != -1) {